Prophase

During early prophase, the chromatin is beginning to condense into chromosomes (black arrows) and the nuclear envelope is mostly complete, though evidence of dissolution is present (blue arrows).  Also, centrosome migration to opposite poles of the nucleus begins during this phase.

By mid-prophase, the replicated chromosomes are clearly evident (black arrows) and demonstrate greater basophilia due to the chromatin condensation.  Dissolution of the nuclear envelope is nearly complete, though in some regions, evidence of its presence is visible (blue arrow)

By late prophase, the chromosomes are fully condensed (black arrows) and the nuclear envelope has completely disappeared.  Though not visible in this image, centrosome migration is complete and mitotic spindle formation has begun.